Type I or insulin-dependent diabetes is an important example of a class of autoimmune diseases where the immune response becomes dysregulated and directed to self antigens expressed in specialized organs. While great progress has been made in defining lymphocyte censoring mechanisms that normally ensure immunological tolerance to self antigens expressed ubiquitously or on cells exposed to normal lymphocyte traffic, it is not known how organ-specific antigens are tolerated and disorders such as diabetes normally avoided. In the proposed research, transgenic mouse that have previously been successfully used to study tolerance to systemic autoantigens will be adapted to focus on the regulation of organ-specific tolerance and autoimmunity. The studies will test whether CD4+ T cells and B cells bearing receptors for surface antigens on pancreatic beta cells are regulated by simply not encountering sufficient beta cell autoantigen, by censoring mechanisms involving physical or functional inactivation (anergy) of the T or B cells, or by regulatory interactions with other lymphoid cells. TCR and Ig gene transgenic mice, containing homogenous populations of CD4+ T cells and B cells bearing well-defined receptors for the protein antigen, hen egg lysozyme (HEL), will be used to track the fate of autoreactive cells in vivo. Transgenic mice expressing HEL in membrane-bound form under control of the beta cell-specific rat insulin promoter will be used to compare the regulation of HEL specific T and B cells in the presence or absence of the organ-specific autoantigen. AIMS 1 and 2, will determine how autoreactive CD4+ T cells and B cells are controlled on a strain background where islet antigens are normally tolerated. The actions of exogeneous stimulation by foreign antigens or superantigens in initiating or sustaining breakdown of tolerance in CD4+ T cells will be determined. The role of islet cell autoantibodies of different isotypes in T cell-mediated insulitis and diabetes will be explored in detail, and the possibility that anergic B cells can prevent autoreactive CD4+ T cells from mounting an islet-cell autoimmune response will be tested. In AIM3, transgenic mice carrying one or more diabetes-susceptibility genes from the NOD strain will be used to localize and define the cellular basis for defects in the tolerance or regulatory mechanisms defined in the previous two aims. These studies should elucidate the basic cellular events that normally prevent diabetes and other organ-specific autoimmune disorders, and identify pathways that become dysregulated as a result of genetic and environmental factors leading to autoimmunity.